Supercapacitor energy storage power supply, as a new type of green and environmentally friendly energy storage device, has the characteristics of large energy storage, high charging efficiency, and long service life. It has great application value in the field of rail transit, especially in trams. With the continuous development of urban rail transit, more and more small and medium-sized cities are choosing trams as their first choice for developing urban public transportation. At present, most tram vehicles adopt a hybrid power supply method of energy storage power and batteries. In this power supply method, the energy storage power supply is directly connected to the main circuit of the train and works in coordination with other power supply equipment. The energy storage power supply can be regarded as a large capacity battery. If a tram is equipped with multiple sets of energy storage power sources as the only power source for the train, and multiple sets of energy storage power sources are simultaneously connected in parallel to the main circuit bus of the train, it is necessary to consider how each set of energy storage power sources can safely and reliably supply power to the train. This article comprehensively considers the operation status of multiple sets of energy storage power sources and trams, proposes a grid connected control method for multiple sets of energy storage power sources, and elaborates on the control structure and scheme design based on the different working conditions that may occur during actual operation of energy storage power sources.
1.Principle of parallel power supply with energy storage power sources
Energy storage power sources are composed of multiple modules with different capacities in series and parallel. When multiple energy storage power sources work together, they can play an increasingly important role as energy buffering links. By controlling the energy storage system to achieve grid connected operation, the power quality of the power grid can be improved.
The train is equipped with a main circuit bus that spans the entire vehicle. Three sets of energy storage power sources are connected in parallel on the main circuit bus to supply power to the vehicle load. When one or two sets of energy storage power sources fail, the vehicle can be powered by the energy storage power source that is still in normal operation. The air conditioning, lighting and other loads on the train can continue to work. This grid connected power supply method can be applied to trams to meet the actual operational needs of vehicles to the greatest extent possible.
Energy storage power sources with different voltages can charge and discharge each other. When multiple groups of energy storage power sources are connected to the grid, there will be a voltage balancing process. Due to the extremely small internal resistance of energy storage power sources, if the voltage difference between each group of energy storage power sources is too large during this process, it will form a large circuit current, which poses a risk of damaging the energy storage power source or other electrical equipment. Therefore, the voltage value of energy storage power sources must be considered in the design. Based on theoretical calculations and past project experience, it is determined that when multiple sets of energy storage power sources are integrated into the main circuit bus of the train for operation, the voltage difference between the energy storage power sources shall not exceed a fixed voltage value (i.e. voltage difference threshold).
2.Grid connected control structure of energy storage power supply
The Vehicle Control Unit (VCU) serves as the control and management unit of the train control management system, and communicates with various subsystems of the train through the Multi functional Vehicle Bus (MVB). It is responsible for completing the logical control, fault diagnosis, and data display functions of the train traction, braking, energy storage power supply, and other systems. The multi energy storage power grid connection control method proposed in this article is mainly implemented by VCU, which determines whether the energy storage power can be connected to the grid based on its voltage value, communication status, and other vehicle conditions.
When a certain group of energy storage power supplies meets the grid connection conditions, the VCU sends an energy storage power supply input command to the command control module, which then drives the energy storage power supply input relay. Finally, the relay controls the corresponding contacts to close, allowing the energy storage power supply to connect to the main circuit bus of the train, achieving the goal of energy storage power supply grid connection. When a group of energy storage power supplies fails or other corresponding conditions of the vehicle are not met, the VCU will control the group of energy storage power supplies to disconnect from the main circuit bus of the train, ensuring that the vehicle can still operate stably. The capacitor management system (CMS), as the control and management unit of the energy storage system, is mainly responsible for monitoring and managing the energy storage power supply, external communication and other functions.
3.Design of grid connected control scheme for energy storage power supply
3.1 Grid connection control principle
The VCU sends an energy storage power supply input command to the instruction control module, which controls the corresponding energy storage power supply to be connected to the grid, diagnoses the communication status between N groups of energy storage power supplies and the VCU, and then processes N groups of energy storage power supplies that need to be put into use into N+1 categories. The classification of grid connection control algorithms is as follows.
Type 1: The communication between the N group energy storage power supply and the VCU is normal. If the VCU fails to issue an input command to the N group energy storage power supply at the same time, the VCU will give a prompt that the energy storage power supply needs to be replenished. The specific processing steps are as follows:
1) Determine whether the absolute value of the difference between the highest and lowest voltage in N groups of energy storage power supplies does not exceed the voltage difference threshold. If so, the VCU sends a command to the control module corresponding to these N groups of energy storage power supplies to enable the energy storage power supply, and the algorithm ends; Otherwise, proceed to step 2.
2) Firstly, remove any K groups of energy storage power sources (assuming the number of energy storage power sources removed is K, 1 ≤ K ≤ N-1), compare the remaining N-K groups according to the method in step 1. If the absolute difference between the highest voltage and the lowest voltage does not exceed the pressure difference threshold, the VCU sends an instruction to the control module corresponding to this N-K group of energy storage power sources to enable the energy storage power supply; After balancing the voltage of the energy storage power supply that has been put into operation, determine whether there is an absolute difference between the voltage value and the balanced voltage value in the remaining K groups of energy storage power supplies that does not exceed the voltage difference threshold（ Δ U) If there is an energy storage power supply, the VCU sends an input command to the corresponding group of energy storage power supplies, and the algorithm ends; Otherwise, proceed to step 3.
3) Follow the algorithm in step 2, increasing the number of excluded groups by 1 each time, and then compare the voltage values of the remaining energy storage power sources. If the algorithm is not yet completed when K=N-1, the VCU sends a command to the control module corresponding to the energy storage power source with the highest voltage value in N groups, and the algorithm ends.
Type 2: In N groups of energy storage power supplies, if there is 1 group with abnormal communication with VCU, remove the group of energy storage power supplies and execute the Type 1 algorithm.
Category 3: In N groups of energy storage power supplies, there are 2 groups with abnormal communication with VCUs. Remove these 2 groups of energy storage power supplies and execute the algorithm according to Category 1. And so on.
Class N+1: In N groups of energy storage power supplies, if there is abnormal communication between N groups and VCU, the VCU will not issue an energy storage power supply input command to any corresponding command control module of the energy storage power supply.
3.2 Grid connection control process
At present, a maximum of three sets of energy storage power sources are used as the sole power source for vehicles in practical projects. From the perspective of practical project applications, taking N=3 as an example, the grid connected control method is introduced.
MVB is used for communication between the energy storage power supply and VCU, and the grid connection control process changes based on the communication status between the three sets of energy storage power supplies and VCU.
When the communication between the three sets of energy storage power supplies and the VCU is normal, their grid connection control process.
When one group of energy storage power supplies has abnormal communication with the VCU, the control process for grid connection of two groups of energy storage power supplies with normal communication with the VCU.
When there are two sets of energy storage power supplies that have abnormal communication with the VCU, the VCU only controls the energy storage power supplies that have normal communication with them to be connected to the grid.
When the communication between the three sets of energy storage power supplies and the VCU is abnormal, the VCU does not control the grid connection of the energy storage power supplies.
Considering the startup time of hardware devices such as VCU, instruction control module, and energy storage power supply, the VCU program performs filtering processing in the early stage of power on and evaluates the effectiveness of the voltage value transmitted by the energy storage power supply. The corresponding energy storage power supply meets the input conditions through the custom function block CMS VoltageJudge.
| Signal Name | Variable Name | Variable type | Definition |
| Communication status between energy storage power supply 1 and VCU | XCMS1-CommFlt | Boolean | Input quantity, 1 represents communication failure, 0 represents communication normal |
| Communication status between energy storage power supply 2 and VCU | XCMS2-CommFlt | Boolean | Input quantity, 1 represents communication failure, 0 represents communication normal |
| Communication status between energy storage power supply 3 and VCU | XCMS3-CommFlt | Boolean | Input quantity, 1 represents communication failure, 0 represents communication normal |
| Voltage value of energy storage power supply 1 | ICMS1_Volt | integer | Input quantity, voltage range 0~1500 V |
| Voltage value of energy storage power supply 2 | ICMS2_Volt | integer | Input quantity, voltage range 0~1500 V |
| Voltage value of energy storage power supply 3 | ICMS3_Volt | integer | Input quantity, voltage range 0~1500 V |
| Does energy storage power supply 1 meet the input conditions | XCMS1Contact-Ctrl | Boolean | Output quantity, 1 represents meeting input conditions, 0 represents not meeting |
| Does energy storage power supply 2 meet the input conditions | XCMS2Contact-Ctrl | Boolean | Output quantity, 1 represents meeting input conditions, 0 represents not meeting |
| Does energy storage power supply 3 meet the input conditions | XCMS3Contact-Ctrl | Boolean | Output quantity, 1 represents meeting input conditions, 0 represents not meeting |
| Energy storage power supply supplementary energy prompt | XNeedCharge | Boolean | Output quantity, 1 indicates power supply prompt, 0 indicates no prompt |
The VCU needs to meet the following conditions when sending an energy storage power supply input command to the command control module: train activation, driver’s cab occupancy, energy storage power supply voltage meets the input conditions, energy storage power supply request input, energy storage power supply fuse is normal, knife switch and isolation switch are in operation position. When the above conditions are met simultaneously and the communication between the energy storage power supply and the command control module is normal, the VCU sends a command to enable the energy storage power supply to be connected to the main circuit bus of the train, providing power for all loads of the train. The input and output quantities of the CMSvoltageJudge function block are shown in Table 1.
3.3 Grid connected control scheme test verification
Conduct grid connected control tests with three sets of energy storage power sources, set a pressure difference threshold of 30 V, and set four working conditions based on the communication status between the energy storage power source and VCU. When a certain set of energy storage power sources has abnormal communication with VCU, VCU will set the voltage value of that set of energy storage power sources to zero.
Condition 1: Activate the vehicle. When all three sets of energy storage power supplies communicate with the VCU normally, record the voltage values of each set of energy storage power supplies at this time, and observe the input status of the three sets of energy storage power supplies. If all three sets of energy storage power supplies are in the input status, the verification is passed. This working condition is executed according to the first category of energy storage power grid connection control algorithm.
Working condition 2: First disconnect the power supply line of energy storage power supply 1, and then activate the vehicle. At this time, there is a communication failure between energy storage power supply 1 and VCU. Record the voltage value of each group of energy storage power supplies at this time, and observe the input status of three groups of energy storage power supplies. If only energy storage power supply 1 is in the non input state, the verification is passed. This working condition is executed according to the second category of energy storage power grid connection control algorithm.
Working condition 3: First disconnect the power supply lines of energy storage power supply 1 and energy storage power supply 2, and then activate the vehicle. At this time, communication between energy storage power supply 1 and energy storage power supply 2 and VCU is faulty. Record the voltage values of each group of energy storage power supplies at this time, and observe the input status of the three groups of energy storage power supplies. If only energy storage power supply 3 is in the input status, the verification is passed. This working condition is executed according to the third category of energy storage power grid connection control algorithm.
Working condition 4: First disconnect the power supply lines of the three sets of energy storage power supplies, and then activate the vehicle. At this time, all three sets of energy storage power supplies and VCU communication have failed. Record the voltage value of each set of energy storage power supplies at this time, and observe the input status of the three sets of energy storage power supplies. If all three sets of energy storage power supplies are in a non input state, the verification is passed. This working condition is executed according to the N+1 class of energy storage power grid connection control algorithm.
| Working condition | Energy storage power supply | Communication status between energy storage power supply and VCU | Energy storage power supply voltage value/V | Result |
| 1 | Energy storage power supply 1 | normal | 843 | input |
| 1 | Energy storage power supply 2 | normal | 856 | input |
| 1 | Energy storage power supply 3 | normal | 860 | input |
| 2 | Energy storage power supply 1 | fault | 0 | Not invested |
| 2 | Energy storage power supply 2 | normal | 800 | input |
| 2 | Energy storage power supply 3 | normal | 820 | input |
| 3 | Energy storage power supply 1 | fault | 0 | Not invested |
| 3 | Energy storage power supply 2 | fault | 0 | Not invested |
| 3 | Energy storage power supply 3 | normal | 860 | input |
| 4 | Energy storage power supply 1 | fault | 0 | Not invested |
| 4 | Energy storage power supply 2 | fault | 0 | Not invested |
| 4 | Energy storage power supply 3 | fault | 0 | Not invested |
The test results of VCU controlling the grid connection of three sets of energy storage power sources are shown in Table 2. The experimental results show that the grid connected control method can automatically determine the timing of each group of energy storage power supply based on the communication status between the energy storage power supply and VCU, as well as other vehicle control conditions. It can also provide prompts when the energy storage power supply needs to supplement energy, which can meet the different operating conditions of vehicles and the actual operational needs of users, greatly improving the power supply redundancy and reliability of the energy storage power supply.
4.Conclusion
As an important component of urban rail transit, trams have been recognized by citizens and governments for their convenience, comfort, and aesthetics. The grid connected control method of multiple energy storage power sources introduced in this article improves the utilization rate of energy storage power sources and the reliability of vehicle power supply, ensuring that each group of energy storage power sources always operates within a reasonable voltage range, prolonging the service life of energy storage power sources, and reflecting the application value of energy storage power sources in actual vehicles.